(1) Field of the Invention:
This invention relates to electromagnetic plunger pumps of the type whose delivery pressure is stabilized, and more particularly to an electromagnetic plunger pump which is capable of keeping its delivery pressure substantially constant when the value of a voltage applied thereto remains in the range of values within the tolerance limits of variations.
(2) Description of the Prior Art:
In electromagnetic plunger pumps of the prior art, a variation in the voltage of the power source has produced a change in their delivery pressure. For example, in an electromagnetic plunger pump of one type, variations in the voltage of the power source produce changes in its delivery pressure which are linear as shown in FIG. 1. More specifically, when the voltage of the power source shows a variation of .+-.15% with respect to a predetermined value, there is a change of about .+-.30% in the delivery pressure of the pump.
When a fluid, such as a fuel oil, is pressurized by a pump and ejected through a nozzle in atomized particles for combustion, the quantity of the fuel oil combusted or the quantity of the delivered fuel oil is proportional to the square root of the pressure. Therefore, a change of as high as .+-.30% of the aforementioned delivery pressure will result in a change in the quantity of the fuel oil combusted of about .+-.15%. The change of this degree is generally undesirable, so that there has been a demand for improving the performance of pumps.
Generally, electromagnetic plunger pumps are driven by power supplied from a commercially available AC power source and it often happens that a variation in voltage of the aforementioned degree is often caused by variations in other loads connected in parallel to the power transmission system for the commercial power source.
In order to keep substantially constant the delivery pressure of an electromagnetic plunger pump, several proposals have hitherto been made. One of such proposals involves the use of an electromagnetic plunger pump of a sufficiently high output not to cause a lowering of its delivery pressure from a predetermined range even if there occurs a reduction in voltage. Such pump is provided with a relief valve mechanism or a pressure control mechanism of the pressure-reducing valve type for suppressing the delivery pressure of the pump when the pump has a high pressure. In another proposal, an electromagnetic plunger pump is provided with a constant-voltage device which operates in combination with pump. In a further proposal, the magnetic path of the electromagnetic coil of an electromagnetic plunger pump is subjected to magnetic saturation so that the electromagnetic plunger may be actuated in a magnetically saturated condition.
Some disadvantages are associated with electromagnetic plunger pumps of these proposals. An electromagnetic plunger pump having a relief valve mechanism or a pressure control mechanism of the pressure-reducing valve type becomes complex in construction and a difficulty is encountered in effecting maintenance. Moreover, production cost is increased and the pump becomes large in size.
An electromagnetic plunger pump having a constant-voltage device has the disadvantage of its production cost becoming high. Therefore, an attempt has been made to use zener diodes to reduce the cost of production by taking the cost of production of a pump itself into consideration. However, in this case too, the price of a pump system inevitably becomes high. Moreover, in a system in which zener diodes are used for converting an AC current that has undergone half-wave rectification into a current of a trapezoidal wave form so as to thereby stabilize the voltage, it is possible to limit the value of the height of a sine wave of a power source current, but the width of the wave undergoes a change due to a change in the height of the wave as a whole which is caused by a variation in voltage. The result of this is that a mean value of the current varies at all times, so that a variation in the power source voltage is not satisfactorily compensated for.
In the system for magnetically saturating the magnetic path, it is necessary to keep the magnetizing force at a high level at all times, and this causes a rise in ampere-turn and a rise in the value of a current flowing into the coil. This results in a rise in temperature which makes it necessary to increase the size of the coil to cope with this situation. Thus, this system also increases production cost and the pump becomes large in size, so that the system is not economically acceptable.